This invention relates to a low-noise and low-vibration level internal combustion engine, and more particularly to a multi-cylinder, reciprocating piston internal combustion engine provided with means for lowering the noise level and vibration level of the engine.
In order to effectively reduce the noise level of a reciprocating piston internal combustion engine, it has already proposed to shorten the piston stroke of the engine and decrease the mass of engine parts which reciprocally move. Such shortening of the piston stroke particularly contributes to reduction of engine vibration, since this necessarily causes connecting rods to be shortened, reducing the force of inertia of reciprocally moving parts to a considerable extent. An engine having thus shortened piston stroke is called "short-stroke engine" in which the piston stroke is usually smaller than bore diameter of an engine cylinder.
However, such shortening of the piston stroke brings about the increase in bore diameter of the cylinder, for the purpose of obtaining the same volume of the cylinder. This causes the increase in longitudinal distance of a multi-cylinder engine, accompanying decrease in height of the body of the engine. Therefore, the flexural rigidity in the longitudinal direction of the engine body is lowered in cooperation with the fact that the engine body is formed of a cylinder block and a cylinder head which have been formed separately and independently .
This lowering in flexural rigidity of the engine body contributes to increase the engine vibration to a considerable extent, causing engine noise level to increase. Moreover, this leads to deterioration in durability of the engine.